lyrics: add lrcmux backend

[f1nniboy]

Description

This PR adds https://lrcmux.dev (repo) as a new lyrics fetcher backend. lrcmux aggregates various different lyrics sources and exposes them via a single API, so it's perfect as a replacement for the currently defunct Musixmatch fetcher. Users can host their own instance if they prefer not to share their queries with a central API.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.00%. Comparing base (c90f42d) to head (5ebd801).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6816      +/-   ##
==========================================
+ Coverage   74.78%   75.00%   +0.21%     
==========================================
  Files         163      163              
  Lines       20966    20982      +16     
  Branches     3302     3304       +2     
==========================================
+ Hits        15680    15737      +57     
+ Misses       4529     4486      -43     
- Partials      757      759       +2     

Files with missing lines	Coverage Δ
beetsplug/lyrics.py	90.46% <100.00%> (+8.19%)	⬆️

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[semohr]

Hi!

It would have been nice and transparent to disclose that you're also the author of lrcmux.

Since the project is still very new (it's only a few days old), I would prefer to postpone both this PR and any upstream integration into beets until the project has had more time to mature. At this stage, it's difficult to assess its long-term viability and whether it will see continued development and maintenance. We tend to be fairly conservative about adding new integrations because we will be responsible for maintaining them going forward.

This isn't a criticism of you or your project. I'm generally in favor of adding useful new functionality, but as things stand today, integrating lrcmux into upstream feels a bit premature. We wouldd be happy to revisit this once the project has had some time to settle in tho.